STRUCTURE OF TETRAAQUACOBALT(II) BROMIDE DIHYDRATE (COBALT BROMIDE HEXAHYDRATE)

Tetraaquacobalt(II) bromide dihydrate, [Co(H2O)4]Br2.2H2O, M(r) = 326.83, monoclinic, C2/m, a = 11.0158 (12), b = 7.1694 (10), c = 6.9111 (10) angstrom, beta = 124.761 (7)-degrees, V = 448.4 (1) angstrom 3, Z = 2, F(000) = 314, D(x) = 2.420 g cm-3, lambda-(Mo K-alphaBAR) = 0.71069 angstrom, T = 296 K, mu = 107.13 cm-1, R = 0.022 for 532 unique reflections having I > 0. The single type of Co ion is octahedrally coordinated by four equivalent water-molecule O atoms at the observed distance 2.081 (2) angstrom and two equivalent Br ions at the observed distance 2.6048 (5) angstrom in an array which, with respect to its angles, departs only slightly from regular octahedral geometry. The cobalt-oxygen-bromine complex was found to manifest rigid-body behavior. The cobalt-oxygen distance corrected for rigid-body motion was found to be 2.088 angstrom; the corrected cobalt-bromine distance, 2.6081 angstrom. Associated with each complex are two water molecules of hydration. Assignment and refinement of the coordinates of the four non-equivalent H atoms permitted detailed analysis of the hydrogen bonding, which involves each of the four non-equivalent H atoms significantly. The structure as determined in this study differs markedly from that presented in the sole published single-crystal study [Stroganov, Andreev, Kozina & Solov'ev (1961). Vestn. Leningr. Univ. Ser. Mat. Fiz. Khim. 16, 114-119]. In particular, the space-group assignment differs and the pattern of bromine-oxygen distances is quite different. Contrary to Stroganov et al., this structure is isomorphic with that of cobalt chloride hexahydrate.